you will need 2 peices of MDF , i used 3/4inch thick x 1 foot x 9 inch
cut around the shelf legs and cut all holes for the speakers
bolt the MDF to the shelf 1st
once the MDF is bolted to the shelf you then have something to bolt your 6x9 speaker to,,
this works very well and only costs about £5 to do it
cover with acoustic carpet if required think this costs around £10-£15 from ebay,

heres 2 pictures before the 6x9 goes in as you can see the MDF gets bolted to the shelf 1st then that gives you a good solid place to bolt the speaker to